In radio communications networks a user equipment is communicating via a radio base station by transmitting data to the radio base station. The data is transmitted using a transmission power. Power control in these radio communications networks has fast closed loop, also known as fast inner power control loop, and open loop power control procedures to control the transmission power of the user equipment within a radio communications network. In the fast inner power control loop in an uplink transmission towards the radio base station of code division multiple access (CDMA) systems, in particular in Wideband Code Division Multiple Access (WCDMA) systems, each user equipment transmits a signal spread in the same frequency band. Signals spread in the same frequency band causes interference for the other user equipments transmitting in the same frequency band. The consequence of the interference is that the powers of all user equipments become coupled in the uplink of CDMA cells. Consequently, if minimum possible transmission powers are used from the user equipments the co channel interference is minimized.
The services carried over these systems typically require a certain Signal to Interference Ratio (SIR), to maintain a stipulated quality of service (QoS), a QoS goal. A fast inner power control loop therefore strives to maintain a target SIR, in order to secure the above QoS goal. The target SIR is set by an outer power control loop that uses the observed block error rate for adjustment, which is much slower than the fast inner power control loop. The target SIR may also be adapted in response to so called Hybrid Automatic Repeat Request (HARQ) retransmissions in future releases of WCDMA.
The performance of the fast inner power control loop is impaired by the fact that control commands sent to the user equipment are quantized with only one bit, signaling a command to increase or decrease the user equipment transmit power in steps of typically 1 dB. There are two main problems associated with using only one bit. First, in case of a large commanded change of signal to interference ratio (SIR) from the outer power control loop, the response of the inner loop may require many power steps and consequently a substantial time before the power settles at the new power level. This problem is amplified by the nonlinear coupling between the inner loop power control loops of user equipments. Secondly, during periods of constant power, the 1 dB quantization makes the user equipment transmit power to perform an oscillation, a limit cycle, with the power commands switching between +1 dB and −1 dB constantly as there is no way to signal that the transmission power should be constant. This increases the variance of the SIR. The increased variance, in turn, requires a slightly ½ dB increased SIR target set point, in order not to end up below the service requirements.